Biogenic gas systems in eastern Qaidam Basin

Several giant biogenic gas fields (with proven gas reserves greater than 25 billion cubic meters) have been discovered in recent years in the Sanhu area of eastern Qaidam Basin. This area has an average surface altitude of around 2800 m, and forms the northern segment of the Qinghai-Tibet Plateau. The biogenic gas fields occur mostly within or adjacent to the depocenter of approximately 3400 m of Quaternary sediments. The gas reservoirs, with burial depth generally less than 1900 m, are unconsolidated sandstones with approximately 24–40% porosity, and are interbedded with mudstones containing on average 0.3% TOC. The occurrence of methanogens in the shallow Quaternary sediments appears to depend on both the sedimentary facies and burial depths, thus most of the biogenic gases in the Sanhu area appears to have derived from the source kitchens in the central sag above a biogenic gas floor at the depth around 1800 m. The key gas system elements for the formation of the giant biogenic gas accumulations include (1) secular low surface temperatures and lake water hypersalinity favor the preservation of suitable organic substrates for biogenic methane generation, (2) well-developed sand and mud interbeds, (3) sufficient cumulative thickness of water-saturated mudstones as caprocks, (4) presence of syndepositional anticlines of Pleistocene and later age, (5) a regional hydrogeological system favoring northeastward gas migration, and (6) ongoing dynamic gas migration and accumulation with abundant gas supply.